Carbon dioxide is a well-known gas, which is present in the atmosphere. It is released to the atmosphere in large amounts by fermentation processes, limestone calcinations, and all forms of combustion processes of carbon and carbon compounds. In the recent decades, the attention in respect of said emission has been rising, because of the environmental problem due to future climate changes via the Greenhouse effect. Consequently, extensive work has been performed over the years in order to develop processes for the removal of carbon dioxide from combustion gases. If possible, a subsequent recovery of carbon dioxide may make those processes economically feasible.
One type of conventional method for the recovery of carbon dioxide from a gaseous source is the absorption method, in which carbon dioxide is absorbed in an absorbing agent. If other gases, such as oxygen, are present in the gaseous source, said other gases may also be absorbed chemically and/or physically. This will be the case if an amine-based agent is used as the absorbing agent.
It is well-known from the prior art that when O2 is present in the carbon dioxide-containing gaseous source, and when alkanolamine is used as the absorbing agent, said O2 will be transferred into the alkanolamine-containing absorbing agent during the absorption procedure. As a consequence an unwanted degradation of alkanolamine as well as corrosion problems will occur due to the presence of O2.
Many prior art documents relate to this problem. EP 1 059 110 discloses a system for recovering absorbate such as carbon dioxide using an alkanolamine absorbent fluid, wherein the loaded absorbent is heated in a two step heating procedure prior to the separation of the absorbate from the absorbent, and wherein the loaded absorbent is deoxygenated after the first heating step and prior to the second heating step. The deoxygenation takes place by means of depressurisation.
In EP 1 061 045 a system for recovering absorbate such as carbon dioxide from an oxygen-containing mixture is described, wherein carbon dioxide is concentrated in an alkanolamine-containing absorption fluid, oxygen is separated from the absorption fluid, and carbon dioxide is steam stripped from the absorption fluid and recovered. In this system, the oxygen is separated from the absorption fluid by passing the carbon dioxide loaded absorbent comprising dissolved oxygen in countercurrent mass transfer contact with oxygen scavenging gas.
In other cases, nitrogen oxides (also named NOx), sulphurous compounds and volatile organics may be present as contaminants in addition to O2 in the gaseous source. These contaminants will also be absorbed chemically and/or physically in the absorbing agent, when an amine-based agent is used as the absorbing agent.
In a conventional plant for production of high purity carbon dioxide, the carbon dioxide is firstly absorbed in an absorbing agent and afterwards the carbon dioxide and the absorbing agent is separated in a stripper column. However, part of the contaminants present in the feed gas is absorbed together with carbon dioxide during the absorption step. When separating the carbon dioxide from the absorbing agent in a subsequent stripper process, part of the absorbed contaminants will also be released in the stripper off gas together with the carbon dioxide. The stripper off gas will further contain N2 and O2 in some amounts.
When producing food grade carbon dioxide or other carbon dioxide applications, where a high purity is required, these contaminants must be removed from the stripper off gas in down stream equipment in order to obtain the required purity. Conventional technology available for removing such contaminants includes scrubbing, oxidation, adsorption and distillation.
The first step of the down stream purification of the stripper off gas is most often an oxidation process. In this oxidation step any NOx's present is oxidised to nitrate, which subsequently may be removed as a liquid phase. Furthermore, if sulphur is present as hydrogen sulphide, this compound is oxidised to free sulphur. Unfortunately, this oxidation requires a large supply of chemicals. Various oxidation agents may be used. In particular, potassium permanganate is widely used. However, this particular chemical is highly hazardous and, furthermore, as potassium permanganate may be used for the production of explosives, it is to be expected that commercial use of this chemical may at some point be forbidden.
In the next step of the down stream purification the carbon dioxide containing gas is passed to a dehydrator. In this dehydrator any water present in the gas is absorbed and thereby removed from the gas stream. However, if any residues of acetaldehyde and/or volatile oxygenates are present in the gas, these compounds are also removed in the dehydrator.
In the last step of the down stream purification, the gaseous carbon dioxide is liquefied in a condenser. In the condenser, it is possible to remove any residues of NO, which may still be present. This, however, is not the case for any residues of NO2. In fact if any NO2 is present when the gas reaches the condenser, or if any NO2 is produced inside the condenser, for example due to oxidation of NO, said NO2 will be transferred to the liquid phase in the condenser and is subsequently almost impossible to remove.
Furthermore, carbon dioxide streams may comprise various amounts of volatile organic compounds, such as benzene, which due to regulatory restrictions and internal standards, must be removed to contents as low as in the range of parts per billion (ppb).
Traditionally, benzene residues in the gaseous stream are removed in a carbon filter in a down stream operating unit, which carbon filter is designed to remove the specific content of benzene in the stream.
However, if the composition of the gaseous stream varies for a specific process, so that the carbon filter is not able to retain all benzene present in the gaseous stream, carbon dioxide comprising too high concentrations of benzene may reach the end product such as soft drinks, which would have a seriously adverse impact in a health risk perspective, but certainly also from a marketing point of view.
Additionally, further means for more efficient purification of gaseous streams is always an ongoing need in the field.